Frequency discriminator



Sept. 2, 1952 p, GRAEF 2,609,447

FREQUENCY DISCRIMINATOR Filed Nov. 3, 1950 INVENTOR R R GRAE F A TTO/ VE V Patented Sept. 2, 1952 UNITED STATE FREQUENCY DISCRIMINATOR I Robert P. Graef, Morristowm N. J., assignor to; Bell Telephone, Laboratories, Incorporated, New. York, N. Y,, a corporation of New York Application November 3, 1950, Serial No. 193,845

19 Claims.

This inventionrelates to wave transmission and more particularly tofrequency discriminators.

An object of the invention is to provide two transmission paths in which the difference between the phase shifts therein will be constant over a range of frequencies.

Another object is to produce a unidirectional outputvoltage which is proportional to, and the sign of which is determined. by, the deviation of an alternating input voltagefrom areference frequency. r

Other objects are to simplify the construction and operation, decrease the cost, and improve the performance of frequency discriminators.

A more specific object is to provide a frequency discriminator which will maintain high discrimination-over a range of frequencies without requiring' the adjustment-of an associated phase shifter. V 1

- As an example of the application of the invention, there is disclosed a frequency discriminator comprising two transmission paths connected at and-schematic in part," of one embodiment of a frequency discriminator embodying the invention.

As shown, the frequency discriminator comprises two transmission paths 2 and 3 connected at their input ends, a hybrid wave-guide junction 4 having a pair of conjugate points to which the other ends of the paths! and 3 are respectively connected, and an adjustable impedance 5 associated with the path 2. Each of the paths 2 and 3 is constituted by a wave guide of the'hollow-pipe their input ends, a hybrid junction having a pair chosen that the phase shifts in the two paths dilfer-bya substantially constant amount equal to an odd integral ,multiple of 1r/2 radians over a considerable range of frequencies. I The two guides will, in generahdiffer in length, in velocity of propagation, and in at least one cross-sectional dimension. ,With this construction the discrimination is maintained at a high value over the entire operating frequency range. 7

It has been customaryheretofore to usein both paths wave guides which have'the same .cross section and the same velocity of propagation. When this is done, however, the phase shifts in the two paths will no longer have the required difference if the input frequency is changed. This is owing to the fact that the difierence in the lengths of the two paths .no longer represents the same fraction of a wavelength. An additional change in phase shift results from the change inivelocity of propagation in wave guides with frequency. Therefore, in microwave frequency discriminators constructed inthis way it is necessary, if high discrimination is to be maintained; to provide an adjustable phase shifter in one or both of the paths for maintaining the required phase difference when the input frequency is varied In the frequency discriminator disclosed herein the need for adjustable auxiliary phase shifters is entirely eliminated. Since one or more adjustable tric mode, usually designated 'IEio.

type having a rectangular cross section. As explained more fully below, the lengths andcut-olf wavelengths of the guides 2 and 3 are so proportioned that their respective phase shifts differ by substantially 1/2 radians, or an odd integral multiple thereof, at the operating frequency f0 and over a considerable range of frequencies on each side thereof.

As indicated by the arrow 1, electromagnetic wave energy of the mid-band operating frequency 10 from a suitable source, not shown, is fed in through the wave guide 8, which is a continuation of the guide 2. In the present example the operating mode is the dominant transverse elec- This mode is chosen because, for a given frequency in, it may be transmitted in a wave guide of minimumcross section. The electric field is polarized in a direction perpendicular to the wider sides of the guide, as shown by the vector E1. Part of the input energy is fed into the end of the guide 3 througha circular orifice 9 in one of the wider sides of the guide 8. e

The adjustable impedance 5 comprises a cylindrical cavity resonator'lll which is inserted between two portions of the guide 2 and coupled thereto by means of the oppositely disposed cir cular orifices l2 and I3. In order to provide maximum sensitivity of frequency discrimination the areas of the orifices 9,12 and I3 are so chosen that substantially equal amounts of energy are supplied to the hybrid junction 4 over the paths 2 and 3. The resonator I0 is tuned to resonate at the operating frequency 'fo by an axial adjustment of the push rod M; to the end of which is securedareflecting piston 15. Y

The hybrid. junction 4 comprises two tandemconnected and axially-aligned sections of rectangular wave guide 11 and (8 of substantially equal lengths, each closed at its outer end by a reflecting plate [9. The other ends of the guides 2 and 3 are connected to the hybrid junction 4 at its center in such away that, at the point of junction, the longitudinal axes of the guides 2, 3 and ll-IB are mutually perpendicular. The longitudinal axes of the guides 2, and lll8 lie in a plane perpendicular to the electric field vectors E1 and E2 in the guides 2' and l'!! 8, and these axes of the guides'3 and li -l8 lie in a plane parallel with the vectors E3 and E2 in the guides 3 and l'i l3.' The guide 2 is connected to one of the narrower sides of the guide ll-!3, and the guide 3 to one of the wider sides. Portions of the side walls of the guide [1-48 are removed to permit the energy from the guides 2 and 3 to enter. A post 23 mounted on the inside of the lower wall of the guide l1l3 at its center and extending into the end of the guide 3 and a transversely disposed plate 24 mounted on one of the narrower walls of the guide 3 near the end thereof and extending part-way across the guide provide impedance matching between the; hybrid junction- :3 and the guides 2 and 3, in accordance with Well- I McGraw-I-Iill' Book Company, New York. The

theory and construction of hybrid junctions for wave guides are discussed in greater detail in United States Patent No. 2,445,896 to W. A Tyrrell, issued July 27, 1948.

The wave energy in the guides i! and it is rectified by means of the crystal detectors 25 and 26, respectively, which are mounted within the guidesnear the outer ends thereof at a proper distance from the reflecting plate It, to provide an effective impedance match and are poled to provide output voltages of opposite polarity. A wire lead 28 is connected to the detector through a by-pass condenser comprising a grounded cylindrical outer electrode 23 and a concentric cylindrical inner electrode 38 conductively connected to the lead 2,8. The electrodes 29 and 3% are separated by a dielectric, which may be air. e

The leads 28 are connected through two substantially equal series-connected resistances 3i and 3-2 which are connected at their junction to an output terminal 34. 'The other output ter minal is connected to a point 36 on one of the guides H and I8 andis grounded as shown at 31. A load 38 of suitable impedance is shown connected to the output terminals 34 and 35.

The operation of the frequency discriminator may be described as follows: The input energy divides into two equal portions which are transmitted along the paths 2 and 3, respectively, and impressed upon conjugate points of the hybrid junction 1 in phase quadrature when the resonator 5 is tuned to the operating frequency in. The portion of the energy entering the junction 4 from the path 2 again divides into equal portions which flow in the arms IT and I8, respectively, in phase with the energy in the path 2. The portion of energy entering the junction 4 from the path 3 also divides into equal portions which flow in the arms I! and [8, respectively. If we consider the energy in the arm I! from the path 3 to be in phase with the energy in the path 3. then the energy in the arm 4- I8 from the path 3 is in phase opposition with the energy in the path 3. When it is tuned to the operating frequency in, the resonator H) introduces no phase shift in the path 2, and the relative phase shift between the energy entering the junctiond from the path 2 and that entering from the path 3 will be 1r/ 2 radians, or an odd multiple thereof. In each of the arms 17 and is the energy from the path 2 will thus be in quadrature with an equal amount of energy from the path 3. The unidirectional output voltages from the detectors 25 and 26 are proportional to the vector sums of the energy in the arms H and I8, respectively, but these voltages are of opposite sign with respect to the ground 37; Therefore, under these circumstances, equal and opposite voltages will appear between the output terminals 34 and 35. If the load 38 is a voltmeter, or other indicating device, it will show a-zero reading.

Now if either the input frequency nor the resonant frequency fr of the resonator I0 is changed, these frequencies will no longer coincide, and the resonator will introduce into the path 2 a phase shift which, near resonance, is proportionalin magnitude to the difference between fd-and fr, and the sign of which depends upon whether it or fr is higher. The quadrature relationship in the arms H and 8 with respect to the' energy from the paths 2 ands is thus upsets In-one arm the angle between the two components becomes acute and the vectrn -sum is increased, 'while in the other arm the angle becomes obtuse and the vector sum is decreased. As a result, the output voltage from one of the detectors 25 and 28 increases, while that from the other decreases, providing a net positive or negsome new frequency In, theresonator I0 is returned to resonate at it. But the output voltage at the terminals 34 and 35 will be zero for an input frequency in only if the paths 2 and 3 have a phase difference of 1/2, or an odd integral multiple thereof; at the new frequency. Heretofore, the required :phase relationship has been maintained by providing an adjustable phase shifter in one or both of the'paths 2 and 3 and adjusting them aS fo is changed. In accordance with the present invention, however the lengths and cut-oif'wavelengths of the two paths 2 and 3 are chosen, as explained below, so that the required phase difference is substantially maintained over a considerable frequency range without resorting to auxiliary phase shifters.

If S1 and S2 are the lengths, respectively. of the two transmission paths and their cut-off Wavelengths in free space are M and A2, respectively, their phase difierence 6 in radians, at the free-space wavelength A, corresponding to any frequency ,f, is given by the expression where 5 The first derivative of expression (1) with'-respect to A-gives the slopeof theversus A characteristic. For the'leastchange of cover the operating frequency range, this slope ismade-zero at the mid-band operating free-space wavelength A0 by setting this derivative equal tozero and substituting Au' for A. This gives the following required relationship between S1 and S A/ML-M 1/ 1 n'-,

From Equations ra d 3 theirequired lengths of 'the two paths are foundto be v Computations have shown that the change in 0 with frequencyis minimized if the longer path has the longer cut-01f wavelength. Thus, if S; is longer than S2, then A1 should be greater than A2. longer wave guide ,shouldbe wider than the shorterowave guide. o

Equations4 and 5 give the required lengths S1 and'Sz, in terms of their cut-ofl wavelengths A1 and A2, of two sections of wave guide in which the phase difference 0 will remain most nearly constant over the widest possible frequency range centered at it. Such a pair of wave guides find useful application in 'a number of transmission devices. When used as the paths 2 and 3 in a frequency discriminatorof the type described above, 0 .is-usually .chosen as an odd integral multiple of 1r/2 radians. When this multiple is chosen as unity, Equations 4 and 5 .become If the cut-off free space wavelength Add a rectangular wave guide is known, its inside width 11 and height b may. readily be found from the formula V and Formula 8 reduces to A=2a (9) from which The height 1) determines the characteristic impedance of the wave guide and is selected to provide the desired impedance. However, if it is desired to prevent the transmission of a higher order mode, there is a maximum permissible height I). For example, if transmission of the TE11 or TEo1 mode is to be prevented, this height cannot exceed a half wavelength in free space at the upper end of the operating frequency band.

As will be shown later, this means that the In the phase discriminator disclosed herein either the path 2 01' the path 3 may be the longer. As shown, the path 2 is the longer. The length S1 of the path 2 is found by adding the distance measured along the longitudinal axis of the guide 2 from the center of the orifice 9 to the orifice l2iand the distance between the orifice l3 and the point where the path. joins the hybrid junction 4. The resonator It introduces no phase shift at resonance, and therefore its diameter is not included in measuring the length of the path 2. The length S2 of the path 3 is measured along the longitudinal axis of the guide from the orifice 9 to the point where the guide joins the junction 4. c

' Before'the lengths S1 and S: can be found it is necessary to choose the corresponding. cutoff wavelengths A1 and A2. A recommended design procedure is to choose A2 sufficiently long to permit the transmission of the lowest frequency in the desiredoperating range and A1 as long as possible without permitting the transmission of an undesired mode or modes of a higher order than the mode to be utilized. It has been found that making A1 and A2 as long as possible minimizes the deviation of 0 from the desired value as the operating frequency in is changed. When A1 and A2 have been selected, the lengths S1 and S2 are found from Formulas 4 and 5, respectively, or, for a phase difference-crew, from Formulas 6 and 7, re-

spectively.

- Indetermining the value of the phase difference 0 to be used in Formulas 4 and 5 allowance must be made for the phase shifts that may be introduced by any impedance matching devices, such; fortexample, as .the post 23 and the plate 24. Thus if 0215 the sum of the phase shifts added-by such-devicesin the path 2 andaa is thetotal phase shift added to the path 3, the'proper value of the phasedifference 0 to be substituted for 0 in Formulas 51 and 5 is given e a o As. an example, if the. discriminator shown is to .operateover a band of frequencies with a midband frequency; In ,of- 9030 megacycles, corresponding to a, mid-band wavelength A2 of, 3.33 centimeters, and utilize the T1310 mode, the wave guide 2 may have a length S1 of 13.822 centimeters and a width of 3.10.2.centimeters, giving a cut-off wavelength A1 of 6.204 centimeters, and the wave guide 3 may have a length S2 of 13.320 centimeters and a width of 2.860 centimeters, corresponding to a cut-ofi wavelength Azi'of 5.720. centimeters. :Withthese dimensions :the: total variation in the 'phasediiference ,9 will not exceed 0.34 degree over a 12 per cent frequency band. With :such va small variation, no auxiliaryphase shifters are .jrequiredto' obtain high discrimination throughout; the band. As a'compar on, if'thewav -euides 2v a 3 are made ,of' the same -width 2 ;860 centimeters, in .accordance-withIprior practice, the corresponding variation in 0 over the same band will be more than 16 degrees, requiring the addition of one or more variable phase shifters to obtain a satisfactorily high discrimination.

What is claimed is:

1. In combination, two transmission paths having lengths S1 and S2, respectively, and cutoff wavelengths A1 and A2, respectively, so proportioned that the difference 0 between the phase shifts therein is substantially constant over a considerable range of frequencies, an adjustable resonant impedance associated with one of said paths, and a hybrid junction having a pair of conjugate points, said paths being connected together at one end and to said points, respectively, at their other ends 2. The combinationin accordance with claim 1 in which said lengths are approximately determined, respectively, by the formulas where r is the ratio of A: to in, and M is the wavelength at the mid-band frequency of said range.

3. The combination in accordance with claim 2. in which each of said pathscomprises a wave guide.

4. The combination in accordance with claim 3 in which said wave guides are rectangular in cross section and the longer of said guides has the greater width.

5. The combination in accordance with claim 1 in which the longer of said paths has the longer cut-off wavelength.

6. The combination in accordance with claim 1 in which 11 is made as long as possible without permitting the transmission of an undesired mode of a higher order than the mode to be utilized, and M is long enough to permit the transmission of the lowest frequency in said range.

7. A frequency discriminator comprising two transmission paths connected at their input ends, a hybrid junction having a pair of conjugate points to which the other ends of'said paths are respectively connected, and an adjustable resonant impedance associated with one of said paths, said paths having lengths S1 and S2, respectively, and cut-off wavelengths M. and A2, respectively, so proportioned that the difference between the phase shifts therein is substantially equal to an odd integral multiple of 1r/2 over a considerable frequency range.

8. A frequency discriminator in accordance with claim 7 in which said lengths are approximately determined, respectively, by the formulas V i r 11. A frequency discriminator in accordance with claim '7 in which the longer of said paths has the longer cut-off wavelength.

12. A frequency discriminator in accordance with claim 7 in which 11 is made as long as possible without permitting the transmission of an undesired mode of a higher order than the mode to be utilized, and M is long enough to permit the transmission of the lowest frequency in said range.

13. A frequency discriminator in accordance with claim I in which said paths are adapted to convey substantially equalamounts of microwave energy to said hybrid junction.

14. A frequency' discriminator in accordance with claim 7 which includes means for matching impedances between said hybrid junction and said paths.

15. A frequency discriminator operable over a considerable range of frequencies comprising two wave guides of the hollow-pipe type conheated at their input ends, a hybrid junction having a pair of conjugate points to which the other ends of said guides are respectively connected, and an adjustable cavity resonator coupled to one ofsaid guides, said guides having different lengths S1 and S2, respectively, and different cut-off wavelengths M and A2, respectively, which approximately satisfy the relationships where r is the ratioof A2 to A1, Rois the wavelength at the. midband frequency of said range, and 0 is equal to an odd integral multiple of 1r/2.

16. A frequency discriminator in accordance with claim 15 in which the longer of said paths has the longer cut-off wavelength.

17. A frequency discriminator in accordance with claim 15 in which said wave guides are rectangular in cross section and the longer of said guides has the greater width.

18. A frequency discriminator in accordance with claim. 15 in which said multiple is unity.

19. A frequency discriminator in accordance with claim 15 which is adapted to operate in the dominant transverse electric mode.

ROBERT P. GRAEF.

REFERENCES CITED I The following references are of record in the file of this patent:

UNITE STATES PATENTS 

